The progress of microelectronics in the last years has led to a miniaturization of field devices and an integration of functionalities, which has brought-about in automation technology an effective and cost-favorable application of energy-saving, integrated, decentralized systems. Thus, not only the measured values are ascertained in the sensors and actuators, but, also, the measured values are already preprocessed and linearized. Furthermore, a self-diagnosis of the sensor or actuator is implemented. Prerequisite for the introduction of these decentralized functionalities in a closed automation concept with “intelligent” sensors and actuators is an increased information- and data-exchange of these decentralized units among one another and with a control system. In automation technology, for this reason, in the last years, a large number of fieldbus systems have come into being, which relate either to company-specific areas of application (e.g. BITBUS, CAN, MODBUS RACKBUS) or are based on an international standard (e.g. HART, PROFIBUS-PA, Foundation FIELDBUS, Ethernet). The large number of fieldbus systems, which are currently used in industrial automation technology and process control technology, is referred to in the following with the generic terms “fieldbusses” or “fieldbus systems”.
Conventional field devices are grid-fed, four-conductor, field devices and include at least two electrical supply lines, or conductors, for energy supply of the field device. Furthermore, two further signal lines are needed as fieldbus, which transmit the measured-value-mapping, measurement signal or other communication data signals between the decentralized units and a control station. In general, the measurement signal or communication data signal is produced and transmitted according to a standard usual therefore, e.g. according to the 4-20 mA current-loop standard, a usual frequency-standard or a digital standard.
Moreover, it is also usual in automation technology, to construct the fieldbus of the field devices in a so-called two-conductor technology and to connect them with one another, so that their communication and energy supply is accomplished via the fieldbus exclusively and simultaneously via a two-wire line, whereby the wiring effort and, thus, the wiring costs of networked, decentralized, automation systems can be lessened.
Examples of such two-conductor field devices, especially two-conductor measuring devices or two-conductor actuating devices, are disclosed in, among others, U.S. Pat. No. 6,014,100.
For historically related reasons and for reasons of intrinsic explosion protection, such two-conductor field devices are predominantly so designed, that a supply current instantaneously flowing in the single line-pair, embodied as an electrical current loop, at an electrical current level value lying between 4 mA and 20 mA, simultaneously also represents the measured value instantaneously produced by the field device or the actuating value instantaneously sent to the field device. In the case of two-conductor field devices, the available input power is significantly limited to, for example, 48 mW. The electronics in the field device must be so designed, that it still works reliably also in the case of a minimum signal current of 4 mA. As a result of this, a problem in the case of such two-conductor field devices is, that the electrical power convertible by the field-device electronics—such being also referred to as “available power”—can strongly fluctuate in a, for practical purposes, unpredictable manner during operation of the two-conductor field device. Taking this situation into consideration, modern two-conductor field devices with a 4-20 mA electrical current loop are, therefore, usually so designed, that their device-functionality implemented by means of a microcomputer provided in the evaluating- and operating-circuit is changeable, and, thus, the power converted in the operating- and evaluating-circuit can be matched to the instantaneously available power.
A suitable matching of the field device electronics to the available power can be achieved, e.g. such as also proposed in U.S. Pat. No. 6,014,100, EP-A 1 174 841 or U.S. Pat. No. 5,416,723, by balancing the power instantaneously converted in the field device with the instantaneously available power, and, indeed, in such a manner, that particular, functional units or circuit components of the operating- and evaluating-circuit of the field device are operated with appropriately variable clock rates, coupled with being, at times, turned off or placed in a sleep mode.
A disadvantage of such a clocked circuit component in the operating- and evaluating-circuit of a two-conductor device is to be seen in the fact that, energy-storing components connected to the outputs or inputs of the circuit component are completely discharged and charged in each clock cycle, since the energy-storing component is discharged via a parasitic discharging current of the temporally inactive circuit component. Through such parasitic, reverse-charging processes and reverse-charging currents of the energy storing component, a large part of the energy fed into the circuit component is lost, for example, by development of heat.